Cogeneration equipment produces power and thermal energy from a common fuel source, generally one that is considered to be a waste product from another process. Topping cogeneration systems generate electricity and use the exhaust for heating. Bottoming cogeneration systems produce heat for industrial processes and use a recovery boiler to generate electricity. Cogenerators and combined heat and power systems (CHP) are used by municipalities, hospitals, universities, oil refineries, paper mills, and wastewater treatment plants. Some CHP equipment uses coal, hydrogen, biomass, natural gas, or solar energy as a primary fuel. Other CHP systems use diesel fuel, digester gas, kerosene, naptha, methanol, ethanol, alcohol, flare gas, or landfill gases. Specifications for cogeneration equipment includes size, installed cost, electrical efficiency, overall efficiency, footprint, emissions, and fuels. Site location, interconnection requirements, unit size, and configuration affect the total cost of a cogeneration system.
Cogeneration equipment includes prime movers such as reciprocating engines, combustion turbines, micro-turbines, backpressure steam turbines and fuel cells. Gas-fired reciprocating engines are used in buildings to achieve energy-efficiency levels approaching 80%. When run on biofuels such as methane, they emit low levels of greenhouse gases and can produce 5 kW to 7 MW of power. Combustion turbines generate electricity from the heat produced by steam, hot water, or thermally-activated equipment such as absorption chillers. This category of cogeneration equipment can produce between 500 kW and 25 MW of electricity. Micro-turbines or microturbines are modular products that can run on waste fuels such as landfill gases. They incorporate advanced materials such as thermal barrier coatings and can produce from 25 kW to 500 kW of electricity. Backpressure steam turbines and fuel cells are also commonly available. A fuel cell uses hydrogen, which is typically isolated from a hydrocarbon source such as natural gas, propane, methanol, or gasoline.
Some cogeneration equipment uses thermally-activated technologies for cooling and dehumidification applications. Examples include heat recovery units (HRU), absorption chillers, and desiccant dehumidifiers. Absorption chillers transfer recovered heat from a prime mover to a heat sink through an absorbent fluid and a refrigerant. To provide cooling, the chiller absorbs and releases water vapor into and out of a lithium bromide solution. Desiccant dehumidifiers use drying agents to remove water from air used to condition a building space. Desiccant materials such as silica gel, activated alumina, and lithium chloride salt are exposed to an air stream with relatively high humidity. Cogeneration equipment such as packaged CHP systems and heat recovery steam generators (HRSG) are also available.